As a basic method of active noise control (ANC), a method called “Filtered-x” is known. However, Filtered-x requires identification of spatial characteristics between a control speaker and an error microphone in advance (i.e., secondary path identification), and cannot be used when environmental characteristics change or when an apparatus cannot be fixed.
Also, an ANC method called a direct method which does not require secondary path identification in advance is known. However, with the direct method, when a reference signal changes abruptly at the time of generation of noise, an input to a control speaker increases transiently, and noise is increased conversely, resulting in unstable control. On the other hand, when parameters (step sizes) for controlling coefficient update amounts of adaptive filters are adjusted to prevent such increase in input, convergence of the adaptive filters requires much time.
As described above, the control stability and the convergence speed of the adaptive filter have a trade-off relationship. For this reason, it is difficult to improve noise reduction efficiency. Therefore, an active noise-reduction apparatus is required to efficiently reduce noise.